INITIAL DETECTION OF [H-3]-LABELED PRAZOSIN ALPHA-1-RECEPTORS IN THE PORCINE PITUITARY NEUROINTERMEDIATE LOBE

Abstract

[3H]Prazosin binding to .alpha.1-adrenergic receptors was investigated in homogenates of tissue from the porcine pituitary neurointermediate lobe. Potent .alpha.1-adrenergic antagonists such as prazosin, WB-4101 [N-[2-(2,6-dimethoxyphenoxy)ethyl]-2,3-dihydro-1,4-benzodioxin-2 methanamine-hydrochloride] and phentolamine displayed high potency in competing for [3H]prazosin specific binding in pituitary tissue. Epinephrine and norepinephrine were the only neurotransmitters which displayed high potency in competing for [3H]prazosin binding sites. There was a strong correlation in the pharmacological properties of the .alpha.1 sites in pituitary tissue and the .alpha.1 sites detected by [3H]prazosin in rat brain tissue. .alpha.-Adrenergic agonists demonstrated significantly higher potency for the .alpha.1-receptor sites in pituitary tissue than in brain tissue preparations. The antagonist ketanserin demonstrated significantly higher potency at .alpha.1 sites in pituitary tissue than in brain tissue. This latter observation has important implications for investigators attempting to use [3H]ketanserin as a selective serotonin receptor ligand in pituitary tissue. These results represent the 1st demonstration of .alpha.1-receptors in mammalian neurointermediate lobe tissue.